Method for discontinuous purification of crude acrylic acid by means of crystallization

ABSTRACT

Crude acrylic acid is purified batchwise by crystallization by a process which comprises at least one purification stage and at least one stripping stage and in which at least the first stripping stage is carried out in a different crystallizer from the first purification stage.

[0001] The present invention relates to a process for the batchwisepurification of crude acrylic acid, which comprises at least onestripping stage and at least one purification stage.

[0002] Acrylic acid is a key chemical which is used, for example, assuch or in the form of its alkyl esters for the preparation of polymersproduced by free radical polymerization.

[0003] Usually, acrylic acid is prepared by gas-phase oxidation ofpropene and/or propane under heterogeneous catalysis (cf. for exampleDE-A 19 508 558, EP-A 293 224, EP-A 257 565 and the literature cited inthese publications).

[0004] The gas mixture leaving the gas-phase oxidation does not, ofcourse, consist of pure acrylic acid but of a gas mixture which containsthe latter and from which acrylic acid has to be isolated.

[0005] The various known methods for isolating acrylic acid from theproduct gas mixture of the gas-phase oxidation are summarized in, forexample, DE-A 19 600 955.

[0006] A common feature of the known separation methods is that thedesired acrylic acid is first isolated from the reaction gas mixtureeither by absorption using a solvent (cf. also DE-A 4 308 087) or bypartial condensation. The resulting absorbate or condensate is thenworked up, as a rule by distillation (with or without the addition of anazeotropic entraining agent) and/or extraction, to give an acrylic acidquality which usually contains, based on its weight, ≧95% by weight,frequently ≧96% by weight or ≧97% by weight, often ≧98% by weight, insome cases ≧99% by weight and in some cases even ≧99.5% by weight ofacrylic acid.

[0007] Acrylic acids of the abovementioned qualities are all to bereferred to in this publication very generally by the term crude acrylicacid. Typically, they contain at least one of the following impurities:water, acetic acid, propionic acid, low molecular weight aldehydes, suchas acrolein, furfural or benzaldehyde, esters of acrylic acid and allylalcohol, maleic anhydride, a process polymerization inhibitor, e.g.phenothiazine and/or N-oxyl radicals and diacrylic acid (the adductformed by

[0008] Michael addition of acrylic acid with itself when mixturescontaining acrylic acid are left to stand).

[0009] When acrylic acid is used, for example for free radicalpolymerization purposes, the majority of the abovementioned impuritiesprove to be troublesome (for example, they produce discolorations of theresulting polymer or influence the induction time, i.e. the time untilthe beginning of the polymerization, in a disadvantageous manner).

[0010] A particularly undesirable impurity proves to be diacrylic acid.Although it generally undergoes free radical copolymerization, it doesso with the disadvantage that it cleaves when the polymer is subjectedto a thermal load and liberates monomeric acrylic acid, which cannot betolerated particularly when the polymer is used in the hygiene sector(keyword: superabsorbers in babies' diapers; these polymers arepartially neutralized polyacrylic acids obtained by free radicalpolymerization).

[0011] Usually, crude acrylic acid is therefore not used as such butonly after further purification to give pure acrylic acid.

[0012] The literature (cf. e.g. EP-A 616 998) proposes carrying out thepurification of crude acrylic acid to pure acrylic acid by batchwisecrystallization. There, the initial crude acrylic acid is separated bythe action of low temperatures, in a first crystallization step, intoacrylic acid crystals having a higher purity than the initial crudeacrylic acid and into a mother liquor having a lower purity than theinitial crude acrylic acid. The crystallization can be carried out bothstatically (the melt is at rest during the crystallization, e.g.plate-type crystallizer or ribbed-tube crystallizer) and dynamically(the melt is agitated during the crystallization, e.g. falling-filmcrystallizer or tube with flow over the whole cross section). Theresulting acrylic acid crystals are melted and, if required, furtherpurified in one or more further crystallization steps carried out insuccession.

[0013] To ensure that the yield is economical, the mother liquorobtained in the first crystallization step is also subjected to at leastone further crystallization step. The crystallization steps in which themelt to be crystallized originates from the acrylic acid crystalsobtained in the first crystallization step are generally referred to aspurification stages. Likewise, the first crystallization step isreferred to as the purification stage. In contrast, all crystallizationsteps in which the melt to be crystallized has originated from themother liquor obtained in the first crystallization step are referred toas stripping stages.

[0014] This means that a batchwise purification of crude acrylic acid bycrystallization usually comprises at least one purification stage and atleast one stripping stage.

[0015] For the batchwise purification of crude acrylic acid, too, theprior art (e.g. EP-A 616 998) recommends carrying out both the one ormore purification stages (i.e. including any additionally usedpurification stages) and at least the first stripping stage in one andthe same crystallizer, i.e. in a single crystallizer. The disadvantageof this procedure is that those fractions of the initial crude acrylicacid which are not subjected to a crystallization in the crystallizerare temporarily stored in containers. During this temporary storage,however, diacrylic acid forms. In a subsequent crystallization stage,the diacrylic acid formed during the temporary storage is separated off,but the diacrylic acid separated off is equivalent to a lower yield ofpure acrylic acid.

[0016] It is an object of the present invention to provide an improvedprocess for the batchwise purification of crude acrylic acid bycrystallization.

[0017] We have found that this object is achieved by a batchwise processfor the purification of crude acrylic acid, comprising at least onepurification stage and at least one stripping stage, wherein at leastthe first stripping stage is carried out in a different crystallizerfrom the first purification stage.

[0018] The present invention is particularly important in that theamount of mother liquor remaining after the first purification stage issubstantially smaller than the amount of initial crude acrylic acid tobe purified by crystallization in the first purification stage.

[0019] Since, however, the dimensioning of the crystallizer used for thefirst purification stage (referred to below as crystallizer A) mustnecessarily be tailored to the amount of initial crude acrylic acid tobe treated by crystallization at this stage, the crystallizer A isusually overdimensioned for further purification, by crystallization, ofthe mother liquor obtained in the first purification stage and it is forthis reason that in practice, when only a single crystallizer is usedfor batchwise purification of initial crude acrylic acid bycrystallization, a procedure is adopted in which a plurality of motherliquors obtained in successively performed first purifcation stages arecollected until the amount of mother liquor originating from firstpurification stages and to be further purified by crystallization isappropriate to the dimension of the crystallizer A (otherwise thespace-time yield of pure acrylic acid which is obtainable usingcrystallizer A is not fully satisfactory).

[0020] In the abovementioned case, the diacrylic acid formation due tothe relatively long time required for temporary storage of mother liquorfrom first purification stages is particularly pronounced in said motherliquor.

[0021] It is therefore expedient according to the invention to use, forthe first stripping stage, a crystallizer (referred to below ascrystallizer B) which is designed for a smaller amount to becrystallized, which is only from 10 to 60, preferably from 20 to 50,particularly preferably from 20 to 40, very particularly from 25 to 35,% by volume of the amount of initial crude acrylic acid to be purifiedby crystallization in the first purification stage.

[0022] According to the invention, both the crystallizer A and thecrystallizer B may be a dynamic or a static crystallizer. It is, ofcourse, also possible for the crystallizer A to be a dynamiccrystallizer and for the crystallizer B to be a static crystallizer (orvice versa).

[0023] It is preferable, according to the invention, if both thecrystallizer A and the crystallizer B are dynamic crystallizers.

[0024] It is particularly expedient if both the crystallizer A and thecrystallizer B are falling-film crystallizers, advantageously tubularfalling-film crystallizers in both cases, as described in EP-A 616 998.

[0025] In this case, a tubular falling-film crystallizer B usedaccording to the invention advantageously contains only from 10 to 60,preferably from 20 to 50, particularly preferably from 20 to 40, veryparticularly preferably from 25 to 35, % of the crystallization tubescontained in the tubular falling-film crystallizer A to be usedaccording to the invention.

[0026] Moreover, the two or more crystallizers A and B to be usedaccording to the invention can be operated in a manner known per se.

[0027] A pair of crystallizers A and B according to the invention is tobe referred to as a crystallizer tandem for the purposes of thisinvention.

[0028] Finally, it should be noted that the novel process is used in anoutstanding manner also when the amount of acrylic acid frozen out inthe first purification stage is only up to 30% by weight of the initialcrude acrylic acid used.

[0029] According to the invention, the crystallizer A can, of course, beused for further purification stages (second, third, etc.) and thecrystallizer B for further stripping stages (second, third, etc.).

[0030] For an identical production capacity of pure acrylic acid, thenovel process is always more advantageous with regard to the diacrylicacid waste than a process which uses only one crystallizer.

[0031] Its advantageousness is evident in particular when both more thanone purification stage and more than one stripping stage are used.

We claim:
 1. A process for the batchwise purification of crude acrylicacid by crystallization, comprising at least one purification stage andat least one stripping stage, wherein at least the first stripping stageis carried out in a different crystallizer (in a crystallizer B) fromthe first purification stage (which is carried out in a crystallizer A).2. A process as claimed in claim 1, wherein both the crystallizer A andthe crystallizer B are dynamic crystallizers.
 3. A process as claimed inclaim 1, wherein both the crystallizer A and the crystallizer B aretubular falling-film crystallizers.
 4. A process as claimed in any ofclaims 1 to 3, wherein the amount to be purified, for which thecrystallizer B is designed, is only from 10 to 60% by volume of theamount to be purified for which the crystallizer A is designed.
 5. Aprocess as claimed in claim 3, wherein the tubular falling-filmcrystallizer B contains only from 10 to 60% of the number ofcrystallization tubes of the tubular falling-film crystallizer A.